Method for manufacturing semiconductor device and laser irradiation apparatus

ABSTRACT

It is an object to achieve continuous crystal growth without optical interference using a compact laser irradiation apparatus. A megahertz laser beam is split and combined to crystallize a semiconductor film. At this point of time, an optical path difference is provided between the split beams to reduce optical interference. The optical path difference is set to have a length equivalent to the pulse width of the megahertz laser beam or more and less than a length equivalent to the pulse repetition interval; thus, optical interference can be suppressed with a very short optical path difference. Therefore, laser beams can be applied continuously and efficiently without energy deterioration.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for manufacturing asemiconductor device by laser irradiation and to a laser irradiationapparatus.

2. Description of the Related Art

Laser irradiation is generally used in manufacturing a semiconductordevice. As one of the reasons, as compared with an annealing methodusing radiant heating or conductive heating, the treatment time can bereduced substantially. As another reason, the laser irradiation does notthermally damage a substrate which is easily deformed by heat, such as aglass substrate.

However, a beam cross section of a laser beam has energy intensitydistribution (hereinafter also referred to as intensity distribution),so that an object to be irradiated could not have been irradiated with alaser beam at a uniform intensity. For example, in the case ofcrystallizing an object to be irradiated by laser annealing, it has beenimpossible to obtain a semiconductor film with a uniform crystallinity.

Correspondingly, there is a technology in which a laser beam is splitinto a plurality of beams, and the split beams are combined, to obtainuniform intensity distribution of a laser beam.

This method makes intensity distribution at beam cross section uniform;however, it also cause interference between the split beams (hereinafteralso referred to as split beams). Therefore, although the originalintensity distribution at the beam cross section is eliminated, otherintensity distribution is newly caused by interference; thus, in thiscase, a surface to be irradiated cannot be irradiated with a laser beamat uniform intensity.

In response to the problem, an optical path difference equivalent to atemporal coherence length (coherence length) or more is provided betweenthe beams split using a retarding plate to suppress the interferencecaused by splitting of the beams (Reference 1: Japanese Patent Laid-OpenNo. 2003-287703).

SUMMARY OF THE INVENTION

Since the optical path difference provided in Reference 1 corresponds tothe coherence length or more, a long optical path difference wasnecessarily provided. For example, in the case of a laser beam emittedfrom a solid-state laser, the coherence length reaches several meters toseveral kilometers; thus, it has been impractical to manufacture anoptical system provided with such an optical path difference.

Further, when an optical path difference is provided between splitbeams, difference in the arrival time of each beam at the surface to beirradiated increases as the optical path difference is longer.Consequently, if the optical path difference provided between the splitbeams are too long, a semiconductor film would be irradiated with a beamwhich is delayed after the semiconductor area melted by irradiation witha beam which is not delayed is solidified. In that case, in a statewhere the crystal growth by irradiation with the split beam which is notdelayed is finished, the next split beam is delivered; thus, acrystalline semiconductor film could not have been continuously grownand large crystal grains can not be formed. Further, in the case ofcrystallizing a semiconductor film by such an irradiation method,thermally discontinuous phenomenon is caused, which is completelydifferent from a process flow in which irradiation is conducted with alaser beam having uniform energy distribution.

In view of the above problems, it is an object the present invention toprovide a small laser beam irradiation apparatus in which beaminterference is prevented while uniforming intensity of a beam crosssection. Further, it is another object of the invention to provide amethod for manufacturing a semiconductor device in which continuouscrystal growth can be achieved in crystallizing an object to beirradiated.

A feature of the present invention is a method for manufacturing asemiconductor device, including splitting a laser beam into a pluralityof split beams, providing an optical path difference between one splitbeam and the other split beam in a set of two arbitrary split beamsselected from the plurality of split beams, and irradiating a commonportion of a semiconductor film with the split beams in differentperiods respectively, thereby crystallizing the semiconductor film.Further, the laser beam is a pulsed laser beam having a pulse width of100 fs to 1 ns at a repetition rate of 10 MHz or more, and the opticalpath difference has a length corresponding to the pulse width of thelaser beam or more and less than a length equivalent to the coherencelength or the pulse repetition interval.

A feature of the present invention is a method for manufacturing asemiconductor device, including splitting a laser beam into at least afirst beam and a second beam, crystallizing a semiconductor film byirradiation a common portion of the semiconductor film with the splitbeams in different periods respectively. Further, an optical pathdifference is provided between a first optical path and a second opticalpath. The optical path difference has a length corresponding to thepulse width of the laser beam or more and less than a lengthcorresponding to the coherence length or the pulse repetition interval.In addition, the laser beam is a pulsed laser having a pulse width of100 fs to 1 ns at a repetition rate of 10 MHz or more.

A feature of the present invention is a method for manufacturing asemiconductor device, including splitting a laser beam into a pluralityof split beams, providing a time lag between one split beam and theother split beam in a set of two arbitrary split beams selected from theplurality of split beams, and irradiating a common portion of asemiconductor film with the split beams in different periodsrespectively, thereby crystallizing the semiconductor film. Further, thelaser beam is a pulsed laser having a pulse width of 100 fs to 1 ns at arepetition rate of 10 MHz or more, and the time lag of times at whichthe split laser beams reach the semiconductor film is a timecorresponding to the pulse width of the laser beam or more and less thana time corresponding to the coherence length or the pulse repetitioninterval.

A feature of the present invention is a method of manufacturing asemiconductor device, including splitting a laser beam having a pulsewidth of 100 fs to 1 ns at a repetition rate of 10 MHz or more into afirst laser beam and a second laser beam, delaying the second laser beamfrom the first laser beam, and irradiating a common portion of asemiconductor film with the first laser beam and the second laser beamin different periods respectively, thereby crystallizing thesemiconductor film. Further, the semiconductor film is irradiated withthe second laser beam while the semiconductor film is melted byirradiation with the first laser beam.

A feature of the present invention is a laser irradiation apparatusincluding a unit for splitting a laser beam emitted from an oscillationsource, which has a pulse width of 100 fs to 1 ns at a repetition rateof 10 MHz or more into a plurality of split beams, a unit for providingan optical path difference between one split beam and the other splitbeam in a set of two arbitrary split laser beams selected from theplurality of split laser beams, and a unit for irradiating a commonportion of an object to be irradiated with the split laser beams indifferent periods respectively. The unit for providing an optical pathdifference provides an optical path difference having a lengthequivalent to the pulse width of the laser beam or more and less thanthe coherence length or the pulse repetition interval.

Note that in this specification, a megahertz laser beam means a laserbeam having an ultrashort pulse with a repetition rate of 10 MHz or moreand a pulse width of 100 fs to 1 ns. Note that to irradiate a commonportion with laser beams does not necessarily means that regions to beirradiated completely correspond to each other, but it is acceptable aslong as the regions to be irradiated with the laser beams share a commonportion.

An excimer laser having a pulse width of 25 ns to 200 ns is often usedas a laser beam for crystallizing a semiconductor film. Such an excimerlaser has somewhat long pulse width, so that it is suitable for aprocess which requires time for rearrangement of atoms likecrystallization, and a crystal grain having a diameter of 100 nm to 300nm approximately can be formed. On the other hand, in the case of usingan ultrashort pulse laser having a pulse width of 1 ns or less, crystalgrains do not grow to be sufficiently large because of the excessivelyshort pulse width, which is inadequate for crystallization.

However, the present inventor found in an experiment that, by increasingthe repetition rate thereby increasing the time for heating thesemiconductor film and continuously supplying energy to the object to beirradiated, a crystal grain of a semiconductor film can be madesufficiently large even when an ultrashort pulse laser is used. That isthe megahertz laser beam having a pulse width of 100 fs to 1 ns with arepetition rate of 10 MHz or more. In the invention, thermal energynecessary for crystallization is continuously supplied to asemiconductor film to crystallize the semiconductor film by using arepetition rate of 10 MHz or more even in the case of using anultrashort pulse width of 100 fs to 1 ns.

In addition, the present inventors found that optical interference issuppressed by providing an optical path difference having a length ofthe pulse width or more. The invention is intended for suppressingoptical interference of a megahertz laser by providing an optical pathdifference which is very short.

An ultrashort pulse laser is used for shape processing utilizing thehigh peak output. However, the laser beam used for shape processing isto form a hole or groove by instantaneously sublimating a part of theobject to be irradiated instead of melting the object to be irradiated.Accordingly, the ultrashort pulse laser is different from a laser beamused for crystallizing a semiconductor film, and a laser beam havinghigher energy than a laser beam used for crystallization is used.

It is a feature of the present invention to provide an optical pathdifference or time lag between split beams. Providing an optical pathdifference and providing a time lag have the same meaning indifferentiating the timing of the split beams reaching a surface to beirradiated. However, in this specification, one expressed in lengthrefers to an optical path difference, and the other expressed in timerefers to a time lag.

In the present invention, a megahertz laser beam is split and an opticalpath difference equivalent to the length between the pulse width of amegahertz laser beam and the coherence length the split beams, therebypreventing interference between the split beams. Alternatively, a timelag corresponding to the pulse width of a megahertz laser beam or morebetween the split beams, thereby preventing interference between thesplit beams. Thus, even a slight optical path difference (time lag)allows a uniform thermal energy distribution of an object to beirradiated without optical interference. Consequently, there areadvantages that an optical system for providing an optical pathdifference can also be reduced in size, and a laser irradiationapparatus can be made smaller.

When the repetition rate is set at 10 MHz or more, heat can becontinuously applied to a semiconductor film, which allows continuouscrystal growth. Further, when the pulse width is 1 ns or less, anoptical path difference between the split beams can be 10 cm or less, sothat an optical system which can be practically designed can bemanufactured. In addition, when the pulse width is 100 fs or more, thestructure of the. laser apparatus can be made simple, which isindustrially advantageous.

In the case of a megahertz laser beam having a repetition rate of 10 MHzor more and a pulse width of 100 fs to 1 ns, when the lengthcorresponding to the pulse width and the coherence length are compared,the length corresponding to the pulse width is shorter. Therefore, thepresent invention is effective because the interference can be reducedusing a very short optical path difference.

Further, in the present invention, an optical path difference betweensplit laser beams is short, so that a semiconductor film can becontinuously irradiated even with split laser beams with lessinterference. Accordingly, a crystal having large grain sizes can beformed by continuously growing a crystal of the semiconductor film. Inother words, when a megahertz laser beam is split to a first beam and asecond beam, the semiconductor film melted by the first beam can beirradiated with the second beam before it is solidified.

In addition, the present invention allows crystallization of asemiconductor film even with a laser beam having an ultrashort pulsewidth of 100 fs to 1 ns by setting the repetition rate at 10 MHz ormore.

Accordingly, in the present invention, an object to be irradiated can beirradiated with a laser beam by which a uniform thermal energydistribution of the object to be irradiated without opticalinterference. In addition, when the object to be irradiated is asemiconductor film, a crystal of the semiconductor film can becontinuously grown during application of split beams, thereby obtaininga uniform crystal with large grain size.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A to 1H are figures for explaining Embodiment Mode 1;

FIGS. 2A to 2C are figures for explaining Embodiment Mode 1;

FIGS. 3A to 3D are figures for explaining Embodiment Mode 2;

FIGS. 4A to 4D are figures for explaining Embodiment Mode 2;

FIGS. 5A to 5I are figures for explaining Embodiment 1;

FIG. 6 is a figure for explaining Embodiment 1;

FIGS. 7A and 7B are figures for explaining Embodiment 2;

FIGS. 8A to 8I are figures for explaining Embodiment 3;

FIGS. 9A to 9G are figures for explaining Embodiment 4;

FIGS. 10A to 10D are figures for explaining Embodiment 5;

FIGS. 11A to 11E are figures for explaining Embodiment 7;

FIGS. 12A to 12D are figures for explaining Embodiment 7;

FIGS. 13A to 13D are figures for explaining Embodiment 7;

FIGS. 14A and 14B are figures for explaining Embodiment 7;

FIGS. 15A to 15F are figures for explaining Embodiment 8;

FIGS. 16A and 16B are figures for explaining Embodiment 8;

FIG. 17 is a figure for explaining Embodiment 6; and

FIGS. 18A and 18B each show a micrographs of a megahertz laser beam.

DETAILED DESCRIPTION OF THE INVENTION Embodiment Mode

Embodiment Modes and Embodiments will be described with reference to thedrawings. However, since the present invention can be embodied in manydifferent modes, it is understood by those skilled in the art that themodes and details can be variously changed without departing from thespirit and scope of the present invention. Therefore, the presentinvention should not be construed as being limited to the description ofthis embodiment mode.

Further, Embodiment Modes 1 and 2 described below can be freely combinedas long as it is practical.

Embodiment Mode 1

FIGS. 1A to 1H show shapes of a beam cross section of a megahertz laserbeam which has been split and supplied to a common portion of an objectto be irradiated in different periods respectively, intensitydistributions and thermal energy distributions. FIG. 1A shows the beamcross section of a megahertz laser beam before splitting, FIG. 1B andFIG. 1C show beam cross sections of megahertz laser beams being split,and FIG. 1D shows the beam cross section of a megahertz laser beamsupplied to the common portion of the object to be irradiated indifferent periods respectively. FIG. 1E shows the intensity distributionof the beam cross section before splitting, FIGS. 1F and 1G shows theintensity distribution of the split beam cross section of the splitbeams, FIG. 1H shows the thermal energy distribution which is suppliedto the common portion applied with the split beams in different periodsrespectively. Here, a megahertz laser beam is split into two beams;however, it may be split into any number of beams as long as it is twoor more. The more the number of the split beams, the more uniform theenergy distribution made by the split beams becomes. Note that theshapes of the beam cross sections shown in FIGS. 1A to 1D, the intensitydistribution shown in FIGS. 1E and IF and the thermal energydistribution shown in FIG. 1H are only examples and the invention is notlimited thereto.

The cross section of the beam 11 is circular, and the intensitydistribution has Gaussian distribution (FIGS. 1A and 1E). The beam 11 issplit into beams 12 and 13, which have semicircular beam cross sections,and the intensity distribution has an axisymmetric Gaussian distributionas divided in half (FIG. 1B, 1C, 1F, and 1G). Then, the split beams 12and 13 are applied to the common portion of the object to be irradiatedin different periods respectively so as to supply a uniform thermalenergy distribution to the common portion of the object to be irradiated(FIGS. 1D and 1H). As described above, when a beam is split and appliedto a common portion of an object to be irradiated in different periodsrespectively, nonuniformity of the energy distribution supplied to thecommon portion of the object to be irradiated can be reduced. However,when the split beams 12 and 13 are applied to the common portion of theobject to be irradiated at the same time, interference is newly caused;thus, as shown in FIGS. 2A and 2B, it is necessary to provide a time lagat which the split beams 12 and 13 respectively reach the object to beirradiated.

FIG. 2A shows a megahertz laser beam having a pulse repetition intervalof 12.5 ns as an example of a megahertz laser beam. As shown in FIGS. 1Ato 1H, the spatial profile of the beam 11 is divided in half to form thesplit beams 12 and 13 (FIG. 2B). Then, as shown in FIG. 2C, the splitbeams 12 and 13 are delayed for a time corresponding to a certainoptical path difference (time lag) in order to reduce the interferencewhen the split beams are applied to the common portion of the object tobe irradiated. The time lag here is to be the pulse width of the beam 11and less than the pulse repetition interval. When the time lag is lessthan the pulse width, the pulses of the split beams 12 and 13 overlap,so that optical interference would be caused. Meanwhile, when the timelag is equivalent to the pulse repetition interval of the beam 11, thepulse periodically delivered from a laser source and a pulse of thedelayed split beam 13 overlap, which causes interference. Further, whenthe time lag is equivalent to the pulse repetition interval of the beam11 or more, an optical path difference of 3 m or more is necessarilyprovided, which is impractical. Therefore, the time lag is required tobe shorter than the pulse repetition interval. Accordingly, by settingthe above time lag for the split beams, the surface to be irradiated canbe irradiated with a uniform laser beam without optical interference.Further, in the case of splitting the beam 11 into 3 or more beams,whichever of a set of two beams arbitrarily selected from the splitbeams has the same relationship as the split beams 12 and 13 describedabove. In other words, as to a set of two split beams arbitrarilyselected from the split beams, one split beam is delayed from the othersplit beam for a time corresponding to the pulse width or more and lessthan pulse repetition interval.

Strictly, the maximum of the time lag to be provided equals the time ofthe pulse repetition interval minus the pulse width. In other words, thetime lag t_(d), the pulse width a, and the pulse repetition interval 1/N(N is the repetition rate of the laser beam) has the relation ofa<t_(d)<1/N−a.

In this specification, a pulse repetition interval refers to 1/N underthe condition that the repetition rate of a megahertz laser beam is N.

When an object to be irradiated is irradiated with the split beam 12,the optical energy is converted into thermal energy. The thermal energydistribution here reflects the intensity distribution of the split beam12. However, in the case where the optical path difference between thesplit beams is overly long, thermal energy supplied to the object to beirradiated begins to scatter before applying the split beam 13, and thethermal energy distribution would change. Accordingly, uniform thermalenergy distribution cannot be obtained even when the split beam 13 isapplied and the thermal energy distribution supplied by applying thesplit beam 13 is combined with the thermal energy distribution suppliedby applying the split beam 12. In other words, the object to beirradiated cannot be uniformly irradiated with a laser beam. Therefore,it is preferable to apply the split beam 13 before the thermal energygenerated by the split beam 12 scatters.

In the present invention, an optical path difference is less than alength equivalent to the pulse width of a megahertz laser beam or moreand less than the length corresponding to the pulse repetition interval;thus, the optical path difference can be very short, and the split beam13 can be applied before the thermal energy of the split beam 12scatters. Consequently, uniform energy can be more reliably supplied tothe surface to be irradiated.

Further, in the case where optical interference is reduced by providingan optical path difference of the coherence length or more as isconventional, since the provided optical path difference is long, theenergy of the split beam 13 decays before the energy of the split beam13 reaches the surface to be irradiated; thus, efficiently the laser cannot be applied sufficiently. Besides, when the optical path differenceis long, the split beam 13 would be applied after the energy supplied bythe irradiation with the split beam 12 is scattered; thus, uniformenergy can not be supplied to the object to be irradiated continuouslywith split beams. For example, in the case of crystallizing asemiconductor by laser irradiation, the split beam 13 is applied afterthe semiconductor melted by irradiation with the split beam 12 issolidified, and a crystal of the semiconductor can not be growncontinuously with the split beams, so that a crystal having large grainscan not be obtained.

In contrast, in the present invention, even when an optical pathdifference is provided between split beams, the optical path differenceis short to have a length equivalent to the pulse width of a megahertzlaser or more, the split beams can be continuously applied with lessenergy decay. Further, energy can be continuously supplied to thesurface to be irradiated with the split beams, and high energy can beefficiently supplied to the surface to be irradiated with less opticalinterference. Consequently, in the case of applying the presentinvention to a crystallization of a semiconductor, continuous crystalgrowth can be realized, and a crystal having large grains can beobtained.

Note that a length corresponding to the pulse width in the presentinvention equals to a value of the pulse width multiplied by thevelocity of light, and a length corresponding to a pulse repetitioninterval equals to value of the pulse repetition interval multiplied bythe velocity of light.

FIGS. 18A and 18B show micrographs of silicon. The silicon iscrystallized by a YVO₄ laser having a pulse width of 15 ps at arepetition rate of 80 MHz from an amorphous silicon. In both FIGS. 18Aand 18B, a YVO₄ laser beam is split and applied to a common portion ofsilicon in different periods respectively, FIG. 18A shows the case ofcrystallization using split beams without time lag, and FIG. 18B showsthe case of crystallization using split beams with a time lag of about25 ps.

In a micrograph shown in FIG. 18A, periodic interference fringes areclearly seen. Meanwhile, in FIG. 18B, a uniform crystal state is seenwithout no interference fringe.

As described above, the present invention makes it possible tocrystallize a semiconductor film with a megahertz laser beam which cansupply a uniform thermal energy distribution by providing a very shortoptical path difference and thus eliminating optical interferencebetween the split beams.

Embodiment Mode 2

In this embodiment mode, a mode in which an object to be irradiated overa glass substrate or a quartz substrate is irradiated with a laser beamwill be described. In the case where an object to be irradiated is overa glass substrate or a quartz substrate, a laser beam which is notabsorbed by the object to be irradiated may be reflected by the backface of the substrate and delivered again to the object to beirradiated. In that case, behavior of the light reflected from the backface of the substrate is necessarily considered when the optical pathdifference is determined to prevent the optical interference. In thisembodiment mode, wetting of the optical path difference in considerationof light reflected by the back face of the substrate will be describedwith reference to FIG. 3A to FIG. 4D. Note that the time lag t_(d) isless than a repetition interval of 1/N of the laser beam. It is becausewhen the pulse repetition is set at the pulse repetition interval of thelaser beam or more, an optical path difference equivalent to 3 m or moreusing a conventional technology, which is impractical. Another reason isthat when the time lag equals the pulse repetition interval of the laserbeam, the pulses emitted periodically from a laser source and thedecayed split pulses overlap, thereby causing interference.

In FIGS. 3A to 4D, one of two split beams produced by splitting amegahertz laser beam is a first laser beam 401, and the other split beamis a second laser beam 402. Alternatively, in the case of dividing themegahertz laser beam into a plurality of beams, arbitrary two laserbeams among the plurality of split beams are used as a set. Further, oneof the two laser beams, which is applied first, is a first laser beam401, and the other split beam applied later is a second laser beam.

In the case of applying the first laser beam 401 and the second laserbeam 402, the conditions for preventing interference between the firstand second laser beams on an object to be irradiated 403 over thesubstrate 404 can be classified into the following three in accordancewith the times t₁ to t₃ in which the first laser beam 401 propagatesthrough the substrate 404. Note that, in this embodiment mode, anamorphous silicon film is used as the object to be irradiated 403, and aglass substrate is used as the substrate 404.

The case in which the second laser beam 402 which is delayed is appliedon the object to be irradiated 403 after the first laser beam 401 passesthrough the object to be irradiated 403, reflects off the back face ofthe substrate 404, and passes through the object to be irradiated 403again will be explained with reference to FIG. 3. FIG. 3A shows a momentat which the object to be irradiated 403 is irradiated with the firstlaser beam 401, and a pulsetip 405 of the first laser beam 401 reachesthe object to be irradiated 403. The time here is t=0.

FIG. 3B shows a state after a lapse of t₁ seconds from the state shownin FIG. 3A. FIG. 3B shows the state where the pulsetip 405 of the firstlaser beam 401 passes through the object to be irradiated 403, reflectsoff the back face of the substrate, and applied again on the object tobe irradiated 403. Here, the time t₁ in which the pulsetip 405 of thefirst laser beam propagates through the substrate 404 is represented ast₁=2nD/c when the thickness of the substrate is D, the refractive indexof the substrate is n, and the velocity of light is c.

Next, FIG. 3C shows a state after a lapse of t₂ seconds from the stateshown in FIG. 3B. FIG. 3C shows the state where a pulse end 406 of thefirst laser beam 401 passes through an object to be irradiated 403, andt₂ is corresponding to the pulse width of the first laser beam 401. Inthe state shown in FIG. 3C, the object to be irradiated 403 keeps phaseinformation of the first laser beam 401.

Next, FIG. 3D shows a state after a lapse of t₃ seconds, which is aphase relaxation time, from the state shown in FIG. 3C. In FIG. 3D, theobject to be irradiated 403 does not keep phase information of the firstlaser beam 401. Therefore, even when the object to be irradiated 403 isirradiated with the second laser beam 402, interference between thefirst laser beam 401 and the second laser beam 402 does not occur.Hereupon, from the time when the object to be irradiated 403 isirradiated with the first laser beam 401 (the state shown in FIG. 3A) tothe time when the object to be irradiated 403 is irradiated with thesecond laser beam 402 (the state shown in FIG. 3D) equals to t₁+t₂+t₃.Therefore, in order to reduce the optical interference between the firstlaser beam and the second laser beam, the time lag between the firstlaser beam 401 and the second laser beam 402 is required to be longerthan the above time t₁+t₂+t₃. Accordingly, the following Equation 1holds.t ₁ +t ₂ +t ₃ <t _(d)   (Equation 1)

Note that since laser pulses are emitted from a laser oscillator at arepetition interval of 1/N seconds (N is the frequency of a laser beam),it is necessary to consider not only the difference between the secondlaser beam 402 and the first laser beam 401, but also the differencebetween the second laser beam 402 and the third laser beam which isemitted after a lapse of 1/N seconds from the first laser beam 401.Correspondingly, it is preferable that the third laser beam emittedafter 1/N seconds from the first laser beam 401 reaches after the phaseinformation due to the second laser beam 402 is eliminated in the objectto be irradiated 403. In other words, the third laser beam is preferablyapplied after the second laser beam 402 reflects off the back face ofthe substrate, irradiates the object to be irradiated 403 again, andpasses through the object to be irradiated 403. In FIG. 3, the time inwhich the first laser beam 401 irradiates the object to be irradiated403, the second laser beam 402 reflects off the back face of thesubstrate, and passes through the object to be irradiated 403 again isexpressed as t_(d)+t₁+t₂+t₃. Here, if the repetition interval 1/N of alaser pulse is longer than the above time t_(d)+t₁+t₂+t₃, the secondlaser beam 402 and the third laser beam does not overlap the object tobe irradiated 403 in time, so that interference between laser beams canbe prevented. In accordance with the above condition, the time lag t_(d)can be expressed by Equation 2 below.t _(d)<1/N−t ₁ −t ₂ −t ₃   (Equation 2)

When Equation 1 and Equation 2 are put together, the condition of thetime lag t_(d) where the interference between the first laser beam 401and the second laser beam 402 can be prevented can be expressed byEquation 3 below.t ₁ +t ₂ +t ₃ <t _(d)<1/N−t ₁ −t ₂ −t ₃   (Equation 3)

Note that a phase relaxation time is a time from when the laser beam isabsorbed by the object to be irradiated, to when the phase informationof the laser beam amounts to 1/e (e is a natural number). Accordingly,the above phase relaxation time t₃ is a time from when the first laserbeam 401 is absorbed by the object to be irradiated 403 to when thephase information amounts to 1/e. However, the time is extremely shortcompared with the pulse width, so that the time may be considered ast₃≈0. Accordingly, Equation 3 can be expressed as in Equation 4.t ₁ +t ₂ <t _(d)<1/N−t ₁ −t ₂   (Equation 4)

In other words, when a time lag t_(d) satisfying the above Equation 3 orEquation 4 is provided between the first laser beam 401 and the secondlaser beam of the split beams, the object to be irradiated can beirradiated with the second laser beam 402 without interference with thefirst laser beam 401 reflected by the back face of the substrate. A timelag (optical path difference) is provided by calculating t_(d) from thevalues of t₁: 2nD/c, t₂: a pulse width, and 1/N: a pulse repetitioninterval. When a megahertz laser beam is split into three or more beams,among the split beams, time lag t_(d) satisfying the above Equation 3 orEquation 4 is provided between a set of two arbitrary split beams,thereby preventing interference.

As described above, in the case of considering the light reflected bythe back face of the substrate, the pulse width t₂ plus the time t₁during which the first laser beam propagates through the substrate maybe used as the minimum value of the time lag (optical path difference).Further, the repetition interval minus the time t₁ during which thefirst laser beam propagates through the substrate as well as the pulsewidth t₂ is to be used as the maximum of the time lag (optical pathdifference).

Using Equation 3 or Equation 4 obtained as above, the time lag t_(d)between the first laser beam and the second laser beam can bedetermined, so that an optical path difference required for an opticalsystem can be calculated. For example, when a pulse width t₂ of a laserbeam emitted from laser oscillator 101 is 10 ps, a phase relaxation timet₃ of the object to be irradiated 403 which is a surface to beirradiated is 0.1 ps, the thickness of a substrate is 1 mm, and therefractive index of the substrate is 1.5; t₁+t₂+t₃=20.1 ps is satisfied.When the time lag t_(d) between the laser beams is 20.1 ps or more inaccordance with Equation 3, optical interference on the surface to beirradiated can be prevented. Further, the maximum of the time lag t_(d)can be calculated from 1/N−t₁−t₂−t₃. In the case where the pulserepetition interval 1/N is 10 ns, the maximum of the delay time is 9.98ns, optical interference can be prevented when the time lag t_(d) is setat 20.1 ps to 9.98 ns.

Note that when an object to be irradiated is crystallized by a megahertzlaser beam, a time lag t_(d) within a set time of the object to beirradiated is preferably provided. When the delay time is set within theset time, the object to be irradiated can be continuously irradiatedwith the first laser beam and the second laser beam, so that continuouscrystal growth using split beams can be realized. Therefore, the timelag t_(d) may preferably be adjusted fitly by a practitioner inaccordance with the melting time of the object to be irradiated in therange of Equation 3 or Equation 4. For example, the set time of siliconis 100 ns, so that a crystal of silicon can be continuously grown bysetting the time lag t_(d) within 100 ns.

Next, another example of preventing interference between the first laserbeam and the second laser beam will be described with reference to FIG.4. The same components as FIG. 3 are denoted by the same marks andsymbols as the FIG. 3.

FIGS. 4A to 4D show the case where the object to be irradiated 403 isirradiated with the second laser beam 402 while the first laser beam 401passes through the object to be irradiated 403 and light reflected bythe back face of a glass substrate irradiates the object to beirradiated 403 again. In FIG. 4A shows a state where the object to beirradiated 403 is irradiated with the first laser beam 401 and the timet=0. Here, a pulsetip 405 of the first laser beam 401 reaches the objectto be irradiated 403.

A time lag between the first laser beam 401 and the second laser beam402 is set as t_(d). A pulsetip 407 of the second laser beam 402 reachesthe object to be irradiated 403 after a lapse of t_(d) seconds. Thestate is shown in FIG. 4B. At this point of time, in order to preventinterference between the first laser beam 401 and the second laser beam402, it is necessary to irradiate the object to be irradiated 403 withthe second laser beam 402 after a pulse end 406 of the first laser beam401 passes through the object to be irradiated 403, and the phaseinformation of the first laser beam 401 is eliminated. Therefore, timelag t_(d) between the first laser beam 401 and the second laser beam 402can be expressed by Equation 5 below using a pulse width of t₂, and aphase relaxation time of t₃.t ₂ +t ₃ <t _(d)   (Equation 5)

FIG. 4C shows a figure of a state after a lapse of the pulse width t₂seconds of the second laser beam 402 from the state shown in FIG. 4B,where a pulse end 408 of the second laser beam 402 passes through theobject to be irradiated 403.

After a lapse of phase relaxation time t₃ from a state shown in FIG. 4C,phase information of the object to be irradiated 403 due to the secondlaser beam 402 is eliminated. The state of this time is shown in FIG.4D. After the phase information of the second laser beam is eliminated,if the first laser beam 401 reflected by the back face of the substrateirradiates the object to be irradiated 403 again, interference betweenthe first laser beam 401 and the second laser beam 402 is not caused.Note that time from when the object to be irradiated 403 is irradiatedwith the pulsetip 405 of the first laser beam 401 (FIG. 4A) to when theobject to be irradiated 403 is irradiated with the pulsetip 405 again(FIG. 4D) is denoted by t₁ as shown in FIGS. 3A to 3D. Therefore,Equation 6 below can be used in order to satisfy the condition underwhich interference does not occur in the states shown in FIGS. 4A to 4D.t _(d) +t ₂ +t ₃ <t ₁   (Equation 6)

When Equation 5 and Equation 6 are put together, the condition where theinterference between the first laser beam 401 and the second laser beam402 can be prevented can be expressed by Equation 7 below.t ₂ +t ₃ <t _(d) <t ₁ −t ₂ −t ₃   (Equation 7)

Finally, a case where the object to be irradiated 403 is irradiated withthird laser beam emitted after a lapse of 1/N seconds from the firstlaser beam 401 while the second laser beam 402 passes through the objectto be irradiated 403 and reflects off the back face of the substrate,and the reflected light irradiates the object to be irradiated 403again. The time from when the object to be irradiated 403 is irradiatedwith the first laser beam 401 (FIG. 4A), to when the object to beirradiated 403 is irradiated with the second laser beam 402, and thephase information of the object to be irradiated 403 due to the secondlaser beam 402 is eliminated (FIG. 4D) is expressed by t_(d)+t₂+t₃.Here, in the case where the repetition interval 1/N of the laser pulseis longer than the above time t_(d)+t₂+t₃, the second laser beam 402 andthe third laser beam do not overlap on the object to be irradiated 403in time; thus, interference between the laser beams can be prevented.With the above relationship, Equation 8 below with respect to the timelag t_(d) is obtained.t _(d)<1/N−t ₂ −t ₃   (Equation 8)

Further, the time during which the object to be irradiated 403 isirradiated with the first laser beam 401, the object to be irradiated403 is irradiated with the third laser beam, and the phase informationof the object to be irradiated 403 due to the third laser beam iseliminated is expressed by 1/N+t₂+t₃. In the case where the second laserbeam 402 reflected by the back face of the substrate reaches the objectto be irradiated 403 after a lapse of the above time 1/N+t₂+t₃, thethird laser beam and the second laser beam do not overlap on the objectto be irradiated 403 in time; thus, interference between the laser beamscan be prevented. Note that the time during which the object to beirradiated 403 is irradiated with the first laser beam 401, the objectto be irradiated is irradiated with the second laser beam, and thesecond laser beam reflected by the back face of the substrate reachesthe object to be irradiated 403 is expressed by t_(d)+t₁. Therefore,when Equation 9 below holds, after the phase information of the thirdlaser beam is eliminated from the object to be irradiated 403, thesecond laser beam reflected by the back face of the substrate irradiatesthe object to be irradiated again, thereby preventing interference.1/N+t ₂ +t ₃ <t _(d) +t ₁   (Equation 9)

When Equation 8 and Equation 9 are put together, the condition where theinterference between the third laser beam emitted after a lapse of 1/Nseconds from the first laser beam and the second laser beam 402 can beprevented can be expressed by Equation 10 below.1/N−t ₁ +t ₂ +t ₃ <t _(d)<1/N−t ₂ −t ₃   (Equation 10)

Note that, phase relaxation time t₃ is a time from when the first orsecond laser beam is absorbed by the object to be irradiated 403 to whenthe phase information becomes 1/e as in FIG. 3. However, the time isextremely short, so that the time may be considered as t₃≈0.Accordingly, Equation 10 can be expressed as in Equation 11 below.1/N−t ₁ +t ₂ <t _(d)<1/N−t ₂   (Equation 11)

In the case of splitting a megahertz laser beam into 3 or more beams,among the split beams, time lag t_(d) satisfying the above Equation 10or Equation 11 is provided between a set of two arbitrary split beams,thereby preventing interference.

Note that when an object to be irradiated is crystallized by a megahertzlaser beam, time lag t_(d) within a set time of the object to beirradiated is preferably provided. When the delay time is set within theset time, the object to be irradiated can be continuously irradiatedwith the first laser beam and the second laser beam, so that continuouscrystal growth using split beams can be realized. Therefore, the timelag t_(d) may preferably be adjusted fitly by a practitioner inaccordance with the melting time of the object to be irradiated in therange of Equation 10 or Equation 11.

A time lag t_(d) is calculated from the Equations above, and an opticalsystem may be designed to use the obtained optical path difference.Thus, an optical path difference can be designed considering thebehavior of light reflected by the back face of the substrate as well.By providing an optical path difference according to the invention, anamorphous silicon film can be irradiated to supply a uniform thermalenergy distribution without optical interference, thereby continuouscrystal growth can be performed. In other words, in order to uniformlyirradiating a semiconductor film without optical interference, it isacceptable as long as two laser beams are not in a semiconductor film,and phase information of two laser beams is not in the semiconductorfilm accordingly.

In this embodiment, time lag t_(d) is explained using time as a unit;however, in the case of converting it into optical path difference, thedelay time may be multiplied by velocity of light.

Embodiments 1 to 8 below can be freely combined as long as it ispractical. Further, Embodiment 1 to 8 below can be freely combined withEmbodiment Mode 1 or 2 as long as it is practical.

Embodiment 1

In Embodiment 1, a laser irradiation apparatus of the present inventionwill be described. FIG. 5A shows a laser irradiation apparatus, FIG. 5Bshows a shape of a beam cross section before splitting, FIGS. 5C and 5Dshow shapes of cross sections of split beams, and FIG. 5E shows a crosssection of the split beams applied to a common portion of a surface tobe irradiated in different periods respectively. FIG. 5F shows intensitydistribution of a cross section of a beam before splitting, FIGS. 5G and5H each show intensity distribution of a cross section split beams, andFIG. 5I shows thermal energy distribution of the surface to beirradiated with the split beams in different periods respectively.

In FIG. 5A, a laser beam 107 emitted from a laser oscillator 101 issplit into a first laser beam 108 and a second laser beam 109 by asplitter. The laser beams 107, 108, and 109 are megahertz laser beams.As the splitter 102 a split mirror is used. The laser beam 107 has acircular beam cross section and intensity of Gaussian distribution(FIGS. 5B and 5F). The first laser beam 108 and the second laser beam109 are split by the split mirror to have semicircular beam crosssections (FIGS. 5C and 5D). The luminous intensities has distributionsof Gaussian distribution divided in half, which are axisymmetric (FIGS.5G and 5H).

The first laser beam 108 and the second laser beam 109 reflect off amirror 104 and a mirror 103 respectively to be polarized, and the firstlaser beam 108 and the second laser beam 109 are applied to the commonportion of the surface to be irradiated in different periodsrespectively. The first laser beam 108 and the second laser beam 109 areapplied to the common portion of the surface to be irradiated indifferent periods respectively so that nonuniformity of the thermalenergy distribution supplied to the common portion of the surface to beirradiated is reduced as shown in FIGS. 5E and 5I.

When an optical path length d1 from the splitter 102 to the mirror 104and an optical path length d2 from the splitter 102 to the mirror 103are compared, d1<d2 is satisfied, and the difference between the opticalpath lengths (d=d2−d1) is an optical path difference between the firstlaser beam 108 and the second laser beam 109. Further, an optical pathdifference d is set to have a length corresponding to the pulse width ofthe laser beam 107 or more and less than a length corresponding to thepulse repetition interval. In this embodiment, since a megahertz laserbeam is used, the optical path difference d may be several mm to severaltens mm, and a long optical path difference is not required to beprovided. For example, when a pulse width is 10 ps, the optical pathdifference d is 3 mm, which is very short optical path difference.

Further, as in Embodiment Mode 2, in the case of considering lightreflected by the back face of the substrate, optical path difference din which t_(d) calculated from the Equation is multiplied by velocity oflight may be set.

Next, an example of using a laser irradiation apparatus in which a halfwave plate and a beam splitter for will be described with reference toFIG. 6. The same components as in FIG. 5A are denoted by the same marks.

A laser beam 107 output from a laser oscillator 101 megahertz laser beamis expanded approximately five times in both a longitudinal directionand a transverse direction by beam expander 201 including sphericallenses 201 a and 201 b. The spherical lens 201 a has a radius of 50 mmand a thickness of 7 mm, and the curvature radius of a first surface is−220 mm. The spherical lens 201 b has a radius of 50 mm and a thicknessof 7 mm, and the curvature radius of a second surface is −400 mm. Notethat the beam expander 201 is advantageous in the case where the laserbeam emitted from the laser oscillator 101 is small; however, it may notnecessarily be used in the case where the laser beam is large, and apractitioner may suitably decide whether or not to use it. In thisspecification, the curvature radius of a lens is positive when thecenter of the curvature is on a beam emitting side of the lens face, andis negative when the center of the curvature is on a beam incident sideof the lens face. In addition, a face of the lens where light enters isthe first surface and a face from which light is emitted is the secondsurface.

The laser beam 107 expanded by the beam expander 201 is reflected by thesplitter 102. The splitter 102 splits the laser beam 107 into two of thefirst laser beam 108 and the second laser beam 109 which havesemicircular cross sections and axisymmetric intensity distributions asshown in FIGS. 5C, 5D, 5G, and 5H.

The first laser beam 108 is reflected by the mirror 104, and passesthrough the half wave plate 203. After the first laser beam 108 passesthrough the half wave plate 203, it is s-polarized. Further, the firstlaser beam 108 enters a polarization beam splitter 205.

Meanwhile, the second laser beam 109 split by the splitter 102 isreflected by the mirror 103, and passes through the half wave plate 202.After the second laser beam passes through the half wave plate 202, itis p-polarized. Further, the second laser beam 109 reflected by themirror 204 enters the polarization beam splitter 205.

An optical path of the second laser beam 109 is longer than an opticalpath of the first laser beam 108 by a distance d which is the distancebetween the mirror 204 and the polarization beam splitter 205, and anoptical path difference d is provided for the second laser beam 109.Therefore, the second laser beam 109 is applied on the surface to beirradiated 110 delayed for a time corresponding to the optical pathdifference d from the first laser beam 108. The optical path differenced has a length corresponding to the pulse width of the laser beam 107 ormore and less than a length corresponding to the pulse repetitioninterval. Further, in the case of considering light reflected by theback face of the substrate, an optical path difference calculated fromthe Equations obtained in Embodiment Mode 2 is provided.

The first laser beam and the second laser beam are applied from thepolarization beam splitter 205 in different periods respectively,thereby supplying a uniform thermal energy distribution to the commonportion of the surface to be irradiated. The first laser beam projectedon the surface to be irradiated 110 by cylindrical lenses 105 and 106.The focal length of the cylindrical lens 105 is 150 mm, and thethickness is 5 mm. Thus, a laser beam can be shaped so that the beamcross section on the surface to be irradiated 110 has a length of 500 μmin a longitudinal direction. Further, the focal length of thecylindrical lens 106 is 20 mm. Thus, a laser beam is shaped so that thebeam cross section on the surface to be irradiated 110 has a length of10 μm in a transverse direction. In the above manner, a linear beam on asurface to be irradiated can be formed.

In this embodiment, a laser beam can be split and applied to the commonportion of the semiconductor film in different periods respectivelywhile suppressing interference between laser beams; thus, the thermalenergy distribution on a surface to be irradiated can be made uniform.Consequently, uniform crystal growth can be realized. Further,continuous crystal growth can be conducted by irradiation with evensplit beams, thereby obtaining a large grain crystal.

For example, a YAG laser having a pulse width of 10 ps, a repetitionrate of 10 MHz is split and applied to a common portion of silicon indifferent periods respectively, thereby crystallizing silicon. The settime of silicon is generally 100 ns. In the case where an optical pathdifference equivalent to the coherence length is provided for the YAGlaser to suppress optical interference; normally, a time lag long as 300ns or more must be provided between the split beams. Therefore, sincetime lag of the split beams is longer than the set time of silicon,silicon can not be continuously crystallized with the split beams. Onthe other hand, in the case of providing an optical path differencecorresponding to the pulse width or more for the YAG laser, time lag ofat least 10 ps or more is caused between the split beams. Since 10 ps ismuch shorter than the set time of silicon, silicon can be continuouslycrystallized with the split beams.

Embodiment 2

In Embodiment 2, a laser irradiation apparatus having a differentstructure from Embodiment 1 will be describe with reference to FIG. 7AFIG. 7A is a top view of a laser irradiation apparatus, and FIG. 7B is aside view thereof. The same components as FIG. 6 are denoted by the samemarks and the description will not be repeated.

A laser beam 107 output from the laser oscillator 101 for emitting amegahertz laser beam is expanded in a longitudinal direction and atransverse direction of the beam cross section by the beam expander 201including the spherical lenses 201 a and 201 b.

The laser beam 107 which has passed through the beam expander 201 issplit by a cylindrical lens array 701. Here an example of splitting itinto two is described; however, a cylindrical lens for splitting it intomore than two may also be used. A cylindrical lens included in thecylindrical lens array 701 has a thickness of 5 mm, and the curvatureradius of the first surface is 40 mm.

Here, the laser beam split into two are a first laser beam 108 and asecond laser beam 109 respectively. Further, only the second laser beam109 is passed through a retarder 702 arranged behind the cylindricallens array 701. The retarder 702 may be anything as long as it has afunction of delaying the second laser beam 109 from the first laser beam108. Here, a quartz plate is used as the retarder 702. When thethickness of the quartz plate to be used is d_(1,) velocity of light isc, and the refractive index of the quartz plate is n; the optical pathdifference between the split laser beams is expressed by d₁(n−1)/c, andoptical path difference can be selected by controlling the thickness ofthe quartz plate.

Other than a quartz plate, a medium which does not absorb a megahertzlaser beam may be used for the retarder 702. For example, glass (BK7),water, a crystal containing fluoride, or the like can be used.Incidentally, quartz does not absorb a megahertz laser beam either.

The first laser beam and the second laser beam are collected afterpassing through a cylindrical lens 703. The cylindrical lens 703 has acurvature radius of 70 mm on a first lens surface and a thickness of 5mm. Thus, the first laser beam and the second laser beam are applied toa common portion of a surface to be irradiated in different periodsrespectively behind the cylindrical lens 703, and a uniform thermalenergy distribution is obtained.

The split beams 108 and 109 are projected on the surface to beirradiated 110 by the cylindrical lenses 105 and 106. The length of thebeam cross section in the longitudinal direction is shaped to 500 μm bythe cylindrical lens 105. Further, the length of the beam cross sectionon the surface to be irradiated is shaped by the cylindrical lens 106.The focal length of the cylindrical lens 106 is 20 mm, and the length ofthe beam cross section on the surface to be irradiated is shaped to 10μm. A linear beam spot can be formed on the surface to be irradiated bythe above optical system.

Note that, in this specification, “linear” does not mean “line”, in astrict sense but a rectangular having high aspect ratio (or an oblong).For example, one having an aspect ratio of 2 or more (preferably, 10 to10000) is called to be linear.

In this embodiment, a method for providing an optical path differenceusing a refractive index caused by a retarder is explained. Further, inthis embodiment, the optical path difference may be provided incombination with a method described in Embodiment 1 in which arefractive index is not used.

In this embodiment, a megahertz laser beam can be split and applied to acommon portion of a surface to be irradiated in different periodsrespectively while suppressing the effect of interference between laserbeams by using a megahertz laser beam having an ultrashort pulse. Thus,a uniform thermal energy distribution can be obtained.

Embodiment 3

In this embodiment, an example of a laser irradiation apparatus forsplitting and a megahertz laser having an ultrashort beam pulse andapplying the split beams to a common portion of a surface to beirradiated in different periods respectively will be described withreference to FIGS. 8A to 8I. The same components as FIG. 5A are denotedby the same marks and the description will not be repeated. FIG. 8Ashows a laser irradiation apparatus, FIG. 8B shows a beam cross sectionof a megahertz laser beam before splitting, FIGS. 8C and 8D show beamcross sections of split laser beams, and FIG. 8E shows cross sections ofsplit beams applied to a common portion of a surface to be irradiated indifferent periods respectively. FIG. 8F shows intensity distribution ofa beam cross section before splitting, FIGS. 8G and 8H show intensitydistributions of split beam cross sections, and FIG. 8I shows thermalenergy distribution of the common portion of the surface to beirradiated with the sprit beams in different periods respectively.

A laser beam 107 output from the laser oscillator 101 is converted intoa second harmonic by a nonlinear optical element 801. The laser beamwhich has passed through the nonlinear optical element 801 is expandedboth in a longitudinal direction and a transverse direction by a beamexpander 201 including spherical lenses 201 a and 201 b. Note that thebeam expander is not necessarily used depending on the size or the likeof a laser beam.

The laser beam 107 expanded by the beam expander passes through one of afirst half wave plate 806 a and a second half wave plate 806 b. When thelaser beam 107 passes thorough the first half wave plate 806 a a half ofthe laser beam 107 is polarized in a first polarization direction, andwhen the laser beam 107 passes through the second half wave plate 806 b,a half of the laser beam 107 is polarized in a second polarizationdirection which is deviated for 90° from the first polarizationdirection. The first half wave plate 806 a and the second half waveplate 806 b are provided so that the shapes of the cross sections of thefirst laser beam 108 and the second laser beam 109 are made semicircularby the polarization beam splitter 802 as shown in FIGS. 8C and 8D, andthat the intensity distributions has Gaussian distribution dividedaxisymmetrically as shown in FIGS. 8G and 8H.

Next, the laser beam 107 is split into the first laser beam 108 and thesecond laser beam 109 by the polarization beam splitter 802. The firstlaser beam 108 passes through the polarization beam splitter 802, andreflects off the mirror 104. Further, the first laser beam enters apolarization beam splitter 804.

Meanwhile, the second laser beam is reflected by the polarization beamsplitter 802 and reflected by the mirror 103.

Here, a retarder 803 is provided between the mirror 103 and thepolarization beam splitter. Here, a quartz plate is used as the retarder803. Accordingly, an optical path difference is caused between the firstlaser beam 108 and the second laser beam 109, so that interference thesurface to be irradiated between the first laser beam and the secondlaser beam can be prevented. The optical path difference can be setfreely by selecting the thickness of the quartz plate. Further, thefirst laser beam and the second laser beam are applied to the commonportion of the surface to be irradiated in different periodsrespectively, so that the energy distribution can be made uniform.

The first laser beam 108 and the second laser beam 109 enter thecylindrical lens 105 and the length in the longitudinal direction of thebeam cross section is shaped, and enters the cylindrical lens 106, sothat the length in a transverse direction of the beam cross section isshaped. Note that, the first laser beam 108 and the second laser beam109 immediately after split by the polarization beam splitter 802 haveintensity distributions on the sections as the intensity distribution ofthe laser beam 107 is divided axisymmetrically as shown in FIGS. 8G and8H. The intensity distribution of the laser beam 107 is corrupted as thebeams propagate to make the section loose. However, when the position ofthe polarization beam splitter 802 and the position of the surface to beirradiated 110 are conjugated with respect to cylindrical lens 105 whichoperates on the longitudinal direction of the beam cross section,thereby solving the problem.

In this embodiment, a method for providing an optical path differenceusing a refractive index caused by a retarder is explained. Further, inthis embodiment, the optical path difference may be provided incombination with a method described in Embodiment 1 in which arefractive index is not used.

In this embodiment, a megahertz laser beam can be split and the splitbeams are applied to the common portion of the surface to be irradiatedin different periods respectively while suppressing the effect ofinterference between megahertz laser beams by using a megahertz laserbeam having an ultrashort pulse. Thus, a uniform thermal energydistribution can be obtained.

Embodiment 4

In this embodiment, a method of applying a laser beam different fromEmbodiment 3 will be explained with reference to FIGS. 9A to 9G. FIG. 9Ashows a laser irradiation apparatus, FIGS. 9B and C show the crosssection shapes of first and second laser beams, FIG. 9D shows a crosssection when applying the first laser beam and the second laser beam toa common portion of a surface to be irradiated in different periodsrespectively, and FIGS. 9E and 9F show intensity distributionscorresponding to FIGS. 9B to 9D respectively and 9G shows a thermalenergy distribution of a common portion of a surface to be irradiatedwith the split beams in different periods respectively.

A laser beam 107 output from the laser oscillator 101 is converted intoa second harmonic by a nonlinear optical element 801. The laser beamwhich has passed through the nonlinear optical element 801 is expandedboth in a longitudinal direction and a transverse direction by a beamexpander 201 including spherical lenses 201 a and 201 b. Note that thebeam expander is not necessarily used depending on the size or the likeof a laser beam.

The laser beam 107 expanded by the beam expander is split into the firstlaser beam 108 and the second laser beam 109 by the polarization beamsplitter 802. The cross sections of the split first laser beam 108 andthe second laser beam 109 are circular and the intensity distributionshave Gaussian distributions (FIGS. 9B, 9C, 9E, and 9F).

The first laser beam 108 passes through the polarization beam splitter802, and reflects off the mirror 104. Further, the first laser beamenters a polarization beam splitter 804.

Meanwhile, the second laser beam is reflected by the polarization beamsplitter 802 and reflected by the mirror 103. Further, the second laserbeam enters the polarization beam splitter 804.

Specifically, the optical path length of the second laser beam 109between the polarization beam splitter 802 and the polarization beamsplitter 804 is set to be longer than the optical path length of thefirst laser beam 108 between the polarization beam splitter 802 and thepolarization beam splitter 804 by 0.5 time or less of the beam diameter.Thus, by applying the first laser beam 108 and the second laser beam 109to the surface to be irradiated in different periods respectively, thethermal energy distribution can be made uniform. In the case ofsplitting the laser beam 107 using the polarization beam splitter 802,the intensity distributions of the first laser beam 108 and the secondlaser beam 109 have Gaussian distributions as shown in FIGS. 9E and 9F.Accordingly, by applying the first and second laser beams to the surfaceto be irradiated in different periods respectively using thepolarization beam splitter 804, the first and second laser beams areapplied to completely the same portion; a uniform thermal energydistribution cannot be obtained. However, as in this embodiment, inoverlapping the surfaces to be irradiated with the first laser beam 108and the second laser beam 109, which have some deviation from eachother, the thermal energy distribution can be made uniform.

Further, the optical path length of the second laser beam 109 betweenthe polarization beam splitter 802 and the polarization beam splitter804 may be set to be longer than the optical path length of the firstlaser beam 108 between the polarization beam splitter 802 and thepolarization beam splitter 804 by 0.5 to 0.7 times as the beam diameter.When an optical path difference which is as 0.5 times as the beamdiameter or more is provided, the thermal energy distribution at a timeof applying the first laser beam and the second laser beam to the commonportion of the surface to be irradiated in different periodsrespectively is not uniform as shown in FIG. 9G However, the area of thebeam cross section can be increased, so that an surface can beirradiated more efficiently. Note that when an optical path differenceas 0.7 times as the beam diameter or more is provided, the energydensity of the beam cross section is weak and part of the surface to beirradiated is not crystallized; thus, the optical path difference ispreferably as 0.5 to 0.7 times as long as the beam diameter.

Embodiment 5

In this embodiment, a method for manufacturing a semiconductor deviceusing the present invention will be described.

A method for manufacturing a semiconductor device will be explained withreference to FIG. 10. First, as shown in FIG. 10A, a base film 71 and anamorphous semiconductor film 72 are stacked in order over an insulatingsubstrate 70. Then, the amorphous semiconductor film 72 is irradiatedwith a megahertz laser beam of the present invention (FIG. 10B). Byuniformly irradiating the amorphous semiconductor film with themegahertz laser beam, the amorphous semiconductor film can becrystallized uniformly, thereby obtaining a crystalline semiconductorfilm 73 having large grain crystals. Accordingly, a thin film transistor(TFT) in which little grain boundaries exist in a carrier transferdirection of the channel can be formed using the crystallinesemiconductor film 73.

After that, the crystalline semiconductor film 73 is etched as shown inFIG. 10C, thereby forming island-shaped semiconductor films 74 to 77.Next, a gate insulating film 78 is formed so as to cover theisland-shaped semiconductor films 74 to 77. The gate insulating film 78can be formed of, for example, silicon oxide, silicon nitride, orsilicon nitride oxide by plasma CVD, sputtering, or the like. Forexample, an insulating film containing silicon with a thickness of 30 nmto 200 nm may be formed by sputtering.

Next, a conductive film is formed over the gate insulating film 78 andetched, thereby forming a gate electrode. After that, impuritiesimparting n-type or p-type conductivity are selectively added into theisland-shaped semiconductor films 74 to 77 using the gate electrode as amask, thereby forming a source region, a drain region, an LDD region,and the like. Through the above-mentioned steps, N-channel transistors710 and 712, and P-channel transistors 711 and 713 can be formed overone substrate (FIG. 10D). Subsequently, an insulating film 714 is formedto protect those transistors. This insulating film 714 may be formed ina single-layer structure or a layered structure with an insulating filmcontaining silicon of 100 to 200 nm thick by plasma CVD or sputtering.For example, a silicon oxynitride film of 100 nm thick may be formed byplasma CVD.

An organic insulating film 715 is formed over the insulating film 714.The organic insulating film 715 is formed from an organic insulatingfilm of polyimide, polyamide, BCB, acrylic, or the like applied by anSOG method. The insulating film 715 is preferably formed with a filmsuperior in flatness in the case where the unevenness due to thin filmtransistors formed over the glass substrate 70 is to be reduced.Moreover, the insulating film 714 and the organic insulating film 715are patterned by a photolithography method to form a contact holetherein that reaches the impurity region.

Next, a conductive film is formed from a conductive material andpatterned, thereby forming wirings 716 to 723. After that, an insulatingfilm 724 is formed as a protective film, thereby completing asemiconductor device shown in FIG. 10D. The method for manufacturing asemiconductor device using a laser annealing method of the presentinvention is not limited to the structure of the thin film transistorsdescribed above. In this embodiment, the crystalline semiconductor filmobtained by irradiation with a megahertz laser beam split and applied indifferent periods respectively is used as an active layer of a TFT.Accordingly, it is possible obtain large grain crystals and the mobilityof the semiconductor device can be increased.

A process in which a metal element is added to an amorphoussemiconductor film may be provided before crystallization with laserlight. As the metal element, nickel (Ni), germanium (Ge), iron (Fe),palladium (Pd), tin (Sn), lead (Pb), cobalt (Co), platinum (Pt), copper(Cu), gold (Au), or the like can be used. When laser irradiation iscarried out after the crystallization by adding the metal element, thecrystal formed through the crystallization using the metal element isrecrystallized as a crystal core. Therefore, the crystallinity of thesemiconductor film can be enhanced compared with the crystallizationprocess of only laser irradiation; thus, roughness of the semiconductorfilm surface after crystallization by laser irradiation can besuppressed. Thus, variations in characteristics of the a semiconductorelement typified by TFT which is to be formed later can be reduced andthe off-state current can be suppressed.

Noted that after adding a metal element and crystallizing by heattreatment, the crystallinity may be enhanced by laser irradiation;alternatively, the crystallization may be performed after adding themetal element without heat treatment.

Although this embodiment mode has described the example of using thelaser irradiation method of the present invention for crystallizing thesemiconductor film, the laser irradiation method of the presentinvention may be applied to activate an impurity element added into thesemiconductor film. In this embodiment, in the case of manufacturing asemiconductor device used for a functional circuit such as a driver or aCPU, a thin film transistor having an LDD region or a thin filmtransistor having a structure in which the LDD region overlaps the gateelectrode can be fitly formed. Further, in order to increase the speedof a semiconductor device, it is preferable to make the thin filmtransistor smaller. Since the n-channel transistors 710 and 712 and thep-channel transistors 711 and 713 has an LDD structure including an LDDregion, it is preferably used for a driver or a CPU.

Embodiment 6

In this embodiment, a method for manufacturing an EL display deviceusing the semiconductor device manufactured in accordance withEmbodiment 5 will be described with reference to FIG. 17.

A pixel electrode 504 is formed to be in contact with a wiring 722. Thepixel electrode 504 is formed by etching a transparent conductive film.A compound of indium oxide and tin oxide, a compound of indium oxide andzinc oxide, zinc oxide, tin oxide, or indium oxide can be used for thetransparent conductive film.

After forming the pixel electrode, a partition wall 505 is formed of aresin material. The partition wall 505 is formed by etching an acrylicfilm or a polyimide film with a thickness of 1 μm to 2 μm so that a partof the pixel electrode 504 is exposed. Note that a black film to serveas a black shielding film (not shown) may be provided appropriatelyunder the partition wall 505.

Next, an EL layer 506 is formed. When a light emitting material of theEL layer 506 is an organic compound, an organic EL element is obtained.When a light emitting material of the EL layer 506 is an inorganiccompound, an inorganic EL element is obtained.

An inorganic EL element is classified into a dispersive inorganic ELelement and a thin-film inorganic EL element depending on its elementstructure. The dispersive inorganic EL element has a light emittinglayer in which particles of the light emitting material is dispersed ina binder. The thin-film inorganic EL element has a light emitting layerformed of a thin film of a fluorescent material. In the light emissionmechanism of both the elements, light can be obtained by collisionexcitation of a base material or the light emission center by anelectron which is accelerated in a high electric field. When aninorganic EL element is formed, an insulating layer in which a lightemitting material is dispersed is preferably provided as an EL layerbetween the pixel electrode 504 and an electrode 507, or a lightemitting layer sandwiched between insulating layers is preferablyprovided between the pixel electrode 504 and the electrode 507. Forexample, zinc sulfide (ZnS) or strontium sulfide (SrS) can be used as alight emitting material. An EL layer of an inorganic EL element can beformed by screen printing, vapor deposition, or the like.

An example of using an organic EL element will be described below.

The EL layer 506 and the electrode (MgAg electrode) 507 are formedcontinuously by vacuum vapor deposition without exposure to theatmosphere. Note that it is desirable to form the EL layer 506 in athickness of 100 nm to 1 μm and the electrode 507 to a thickness of 180nm to 300 nm (typically, 200 nm to 250 nm). The EL layer may be formedby an ink-jet method, screen-printing, or the like as well.

In this step, an EL layer and a cathode are sequentially formed in eachpixel corresponding to red, green, and blue. However, it is necessary toform the EL layer individually for each color without using aphotolithography technique because the EL layer has low resistance tosolutions. Therefore, it is preferable to cover pixels other than thepredetermined pixels with a metal mask to form an EL layer and a cathodeselectively in necessary portions. A triplet compound is used for atleast one of each color. Since the triplet compound has higher luminancethan a singlet compound, it is preferable that a triplet compound isused to form a pixel corresponding to red which looks dark, and asinglet compound is used to form other pixels.

In other words, a mask for covering all pixels other than the pixelscorresponding to red is prepared, and an EL layer for red emission andan electrode are selectively formed with the use of the mask. Next, amask for covering all pixels other than the pixels corresponding togreen is prepared, and an EL layer for green emission and an electrodeare selectively formed with the use of the mask. Then, a mask forcovering all pixels other than the pixels corresponding to blue isprepared, and an EL layer for blue emission and an electrode areselectively formed with the use of the mask. Note that different masksare used for each color here; however, one mask may be used for all thecolors. In addition, it is preferable to keep vacuum until the EL layersand electrodes are formed in all the pixels.

Note that the EL layer 506 may be formed of a known material. It ispreferable to use an organic material as a known material inconsideration of drive voltage. For example, an EL layer may have afour-layer structure of a hole-injecting layer, a hole-transportinglayer, a light emitting layer, and an electron-injecting. A film inwhich molybdenum oxide and A-NPD are mixed may also be used for the ELlayer. Alternatively, a hybrid layer in which an organic material and aninorganic material are combined may also be used for the EL layer. Inthe case of using an organic material for the EL layer, each of a lowmolecular weight material, an intermediate molecular weight material,and a high molecular weight material can be used. In addition, thisembodiment mode shows an example of using an MgAg electrode as a cathodeof the EL element; however, another known material may also be used.

The electrode 507 is completed through the steps up to the formation ofa light emitting element 508. Thereafter, a protective film 509 isprovided so as to cover the light emitting element 508 completely. Theprotective layer 509 can be formed to have a single layer or a stack ofinsulating films such as a carbon film, a silicon nitride film, or asilicon nitride oxide film.

Further, a sealing material 510 is provided to cover the protective film509, and a cover member 511 is attached thereto. The sealing material510 is an ultraviolet light curable resin, which is preferably amaterial containing a hygroscopic substance or an antioxidant substance.Furthermore, in this embodiment mode, a glass substrate, a quartzsubstrate, or a plastic substrate can be used for the cover member 511.Although not shown, a polarizing plate may be provided between thesealing material 510 and the cover member 511. The polarizing plate isprovided; thus, high-contrast display can be provided.

In this manner, an active matrix EL display device having a structureincluding a p-channel transistor 711 and an n-channel transistor 710which are included in a driver circuit area, a n-channel transistor 712for switching included in a pixel area, and a p-channel transistor 713for current control is completed. In this embodiment mode, a TFTstructure having an LDD region which does not overlap a gate electrodeis described; however, the TFT is not limited to this structure. The TFTdoes not necessarily have an LDD region and it may have an LDD regionwhich partially or wholly overlap a gate electrode.

Through the above steps, an EL display device can be manufactured.

Embodiment 7

This embodiment mode will describe a method for manufacturing a thinfilm integrated circuit or a contactless thin film integrated circuitdevice (also referred to as a wireless chip, a wireless IC tag, or RFID(Radio Frequency IDentification)), by using a laser irradiationapparatuses shown in one of FIGS. 5A to 9G will be described withreference to FIGS. 11A to 11E.

First, a release layer 1701 is formed over the glass substrate (firstsubstrate) 1700 by sputtering. The release layer 1701 can be formed bysputtering, low-pressure CVD, plasma CVD, or the like. In thisembodiment mode, the release layer 1701 is formed of amorphous siliconto a thickness of approximately 50 nm by sputtering. The material of therelease layer 1701 is not limited to silicon and a material which can beselectively etched away (such as W or Mo) may be used. The thickness ofthe release layer 1701 desirably ranges from 50 nm to 60 nm.

Next, the base insulating film 1702 is formed over the release layer1701. The base insulating film 1702 is formed in order to prevent analkaline earth metal or an alkali metal such as Na contained in thefirst substrate from diffusing into the semiconductor film. An alkalimetal or an alkaline earth metal causes an adverse effect onsemiconductor characteristics if such metal is in the semiconductorfilm. The base insulating film 1702 also has a function to protectsemiconductor elements in a later step of peeling the semiconductorelements. The base insulating film 1702 may have a single layerstructure or a layered structure. Therefore, the base insulating film1702 is formed from an insulating film of silicon oxide, siliconnitride, silicon oxide containing nitrogen (SiON), silicon nitridecontaining oxygen (SiNO), or the like which can suppress the diffusionof alkali metal and alkaline earth metal into the semiconductor film.

Next, an amorphous semiconductor film 1703 is formed over the baseinsulating film 1702. The amorphous semiconductor film 1703 is desirablyformed without exposing the substrate to the atmosphere after formingthe base insulating film 1702. The thickness of the amorphoussemiconductor film 1703 is set in the range of 20 nm to 200 nm(desirably 40 nm to 170 nm, more preferably 50 nm to 150 nm).

Then, the substrate is irradiated with a megahertz laser beam using thelaser irradiation apparatus shown in FIGS. 5A to 9G, thereby uniformlycrystallizing the amorphous semiconductor film 1703. Thus, a crystallinesemiconductor film 1704 is formed (FIG. 11A).

Next, as shown in FIG. 11B, a crystalline semiconductor film 1704 isetched to form island-shaped semiconductor layers 1705 to 1707, and thena gate insulating film 1708 is formed. The gate insulating film 1708 canbe formed with silicon nitride, silicon oxide, silicon oxide containingnitrogen, or silicon nitride containing oxygen in a single-layerstructure or a layered structure by plasma CVD, sputtering, or the like.

After forming the gate insulating film 1708, heat treatment at 300° C.to 450° C. for 1 to 12 hours may be performed in an atmospherecontaining 3% to 100% of hydrogen to hydrogenate the island-shapedsemiconductor layers 1705 to 1707. As another means for thehydrogenation, plasma hydrogenation (using hydrogen excited in plasma)may be conducted.

Next, as shown in FIG. 11C, gate electrodes 1709 to 1711 are formed.Here, after forming Si and W into a stack by sputtering, etching iscarried out using resist masks 1712 as masks, thereby forming the gateelectrodes 1709 to 1711. The conductive material, structure, andmanufacturing method of the gate electrodes 1709 to 1711 are not limitedto these, and can be selected appropriately. For example, a multilayerstructure including NiSi (nickel silicide) and Si doped with impuritiesimparting n-type conductivity (such as phosphorus or arsenic), or alayered structure including TaN (tantalum nitride) and W (tungsten) maybe used. Moreover, a single layer structure using various conductivematerials may be employed. In the case of forming the gate electrodesand an antenna at the same time, the material may be selected inconsideration of their functions.

A mask of silicon oxide or the like may be used instead of the resistmask. In this case, a step of forming a mask of silicon oxide, siliconoxide containing nitrogen, or the like (this mask is referred to as ahard mask) due to etching is added; however, since the decrease in filmthickness of the hard mask by the etching is less than that of a resistmask, the gate electrodes 1709 to 1711 with desired widths can beformed. Moreover, the gate electrodes 1709 to 1711 may be formedselectively by a droplet discharge method without using the resist 1712.

Next, as shown in FIG. 11D, the island-shaped semiconductor layer 1706to be a p-channel TFT is covered with a resist 1713, and theisland-shaped semiconductor layers 1705 and 1707 are doped with animpurity element imparting n-type conductivity (typified by P(phosphorus) or As (arsenic)) using the gate electrodes 1709 and 1711 asmasks. In the doping process, doping is conducted through the gateinsulating film 1708 and a pair of low-concentration impurity regions1716 and a channel region 1719 a are formed in the island-shapedsemiconductor layers 1705; meanwhile, a pair of low-concentrationimpurity regions 1717 and a channel region 1719 b are formed in theisland-shaped semiconductor film 1707. This doping process may beconducted without covering the island-shaped semiconductor layer 1706 tobe a p-channel TFT with the resist mask 1713.

Next, as shown in FIG. 11E, after removing the resist mask 1713 byashing or the like, resists 1718 are newly formed so as to cover theisland-shaped semiconductor layers 1705 and 1717 to be n-channel TFTs.By using the gate electrode 1710 as a mask, the island-shapedsemiconductor layer 1706 is doped with an impurity element impartingp-type conductivity (typified by B (boron)). In the doping process,doping is conducted through the gate insulating film 1708, therebyforming a pair of p-type high-concentration impurity regions 1720 and achannel region 1917 c in the island-shaped semiconductor film 1706.

Subsequently, as shown in FIG. 12A, after removing the resists 1718 byashing or the like, an insulating film 1721 is formed so as to cover thegate insulating film 1708 and the gate electrodes 1709 to 1711.

After that, the insulating film 1721 is partially etched by an etch-backmethod, thereby forming sidewalls 1722 to 1724 that are in contact withboth side walls of the gate electrodes 1709 to 1712 in a self-alignedmanner as shown in FIG. 12B. As the etching gas, a gas mixture of CHF₃and He is used.

Next, as shown in FIG. 12C, a resist 1726 is newly formed so as to coverthe island-shaped semiconductor layer 1706 to be a p-channel TFT. Byusing the gate. electrodes 1709 and 1711 and the sidewalls 1722 and 1724as masks, an impurity element imparting n-type conductivity (typically Por As) is added. In the doping process, doping is conducted through thegate insulating film 1708, thereby forming a pair of n-typehigh-concentration impurity regions 1727 and 1728 in the island-shapedsemiconductor layers 1705 and 1707.

Next, after removing the resist mask 1726 by ashing or the like, theimpurity regions may be thermally activated. At this time, for example,after forming a 50 nm thick silicon oxide film containing nitrogen, heattreatment may be conducted under a nitrogen atmosphere at 550° C. forfour hours. Moreover, after forming a 100 nm thick silicon nitride filmcontaining hydrogen, heat treatment may be conducted under a nitrogenatmosphere at 410° C. for one hour to decrease the defects in thepolycrystalline semiconductor film. Through such heat treatments, defectin a polycrystalline semiconductor film can be reduced. This is, forexample, to terminate dangling bonds in the polycrystallinesemiconductor film and referred to as a hydrogenation treatment processor the like.

Through the above steps, an n-channel TFT 1730, a p-channel TFT 1731,and an n-channel TFT 1732 are formed. In the above manufacturingprocess, fine TFTs each having an LDD length of 0.2 μm to 2 μm can beformed by fitly changing the condition of the etch-back method to adjustthe size of the sidewalls. Moreover, after that, a passivation film forprotecting the TFTs 1730 to 1732 may be formed.

Subsequently, as shown in FIG. 12D, a first interlayer insulating film1733 is formed so as to cover the TFTs 1730 to 1732. Moreover, a secondinterlayer insulating film 1734 is formed over the first interlayerinsulating film 1733. A filler may be mixed into the first interlayerinsulating film 1733 or the second interlayer insulating film 1734.Thus, the first interlayer insulating film 1733 or the second interlayerinsulating film 1734 can be prevented from being peeled or cracked dueto stress caused by difference in coefficient of thermal expansionbetween the first interlayer insulating film 1733 or the secondinterlayer insulating film 1734 and a conductive material and the likefor forming a wiring later.

Next, as shown in FIG. 12D, contact holes are formed in the firstinterlayer insulating film 1733, the second interlayer insulating film1734, and the gate insulating film 1708, and then wirings 1735 to 1739to be connected to the TFTs 1730 to 1732 are formed. The wirings 1735and 1736 are connected to the high-concentration impurity regions 1727of the n-channel TFT 1730, the wirings 1736 and 1737 are connected tothe high-concentration impurity regions 1720 of the p-channel TFT 1731,and the wirings 1738 and 1739 are connected to the high-concentrationimpurity regions 1728 of the n-channel TFT 1732, respectively. Thewiring 1739 is also connected to the gate electrode 1711 of then-channel TFT 1732. The n-channel TFT 1732 can be used as a memoryelement of a random ROM.

Next, as shown in FIG. 13A, a third interlayer insulating film 1741 isformed over the second interlayer insulating film 1734 so as to coverthe wirings 1735 to 1739. The third interlayer insulating film 1741 isformed so that the wiring 1735 is partially exposed. The thirdinterlayer insulating film 1741 can be formed of the same material asthe first interlayer insulating film 1733.

Next, an antenna 1742 is formed over the third interlayer insulatingfilm 1741. The antenna 1742 is formed of a conductive metal of Ag, Au,Cu, Pd, Cr, Mo, Ti, Ta, W, Al, Fe, Co, Zn, Sn, Ni, or the like, or ametal compound thereof. The antenna 1742 is connected to the wiring1735. In FIG. 13B, the antenna 1742 is directly connected to the wiring1735; however, the wireless IC tag of the present invention is notlimited to this structure. For example, using a separately-formedwiring, the antenna 1742 may be electrically connected to the wiring1735.

The antenna 1742 can be formed by a photolithography method, anevaporation method, a droplet discharge method, or the like. In FIG.13A, the antenna 1742 is formed from a single conductive film; however,the antenna 1742 can be formed by stacking a plurality of conductivefilms. For example, the antenna 1742 may be formed by coating a Niwiring or the like coated with Cu by electroless plating. The dropletdischarge method is a method in which a predetermined pattern is formedby discharging a droplet containing a predetermined composition from apore, and includes an ink-jet method and the like in its category. Theprinting method includes screen printing, off-set printing, and thelike. By using a printing method or a droplet discharge method, theantenna 1742 can be formed without using a mask for light exposure.Moreover, a droplet discharge method and a printing method does notwaste materials which are to be etched away in a photolithographymethod. Further, since an expensive mask for light-exposure is notnecessary, the cost spent on the manufacturing of wireless IC tags canbe suppressed.

In the case of using a droplet discharge method or various printingmethods, for example, a conductive particle of Cu coated with Ag or thelike can also be used. If the antenna 1742 is formed by a dropletdischarge method, it is desirable to perform a treatment for increasingthe adhesiveness of the surface of the third interlayer insulating film1741 with the antenna 1742. As a method for increasing the adhesiveness,specifically, a method in which a metal or a metal compound whichincreases the adhesiveness of a conductive film or an insulating film bya catalytic action is attached to the surface of the third interlayerinsulating film 1741, a method in which an organic insulating film, ametal, or a metal compound which has high adhesiveness with a conductivefilm or an insulating film to be formed is attached to the surface ofthe third interlayer insulating film 1741, a method in which surfacemodification is carried out through a plasma treatment on the surface ofthe third interlayer insulating film 1741 under atmospheric pressure orreduced pressure.

If the metal or the metal compound to be attached to the thirdinterlayer insulating film 1741 has electric conductivity, sheetresistance thereof is controlled so that normal operation of the antennais not interrupted. Specifically, an average thickness of the metal orthe metal compound having electric conductivity may be controlled so asto range from, for example, 1 nm to 10 nm, or the metal or the metalcompound may be insulated wholly or partially by oxidation.Alternatively, the attached metal or metal compound may be selectivelyetched away except for a region where the adhesiveness is to beincreased. Moreover, the metal or the metal compound may be selectivelyattached only to a particular region by a droplet discharge method, aprinting method, a sol-gel method, or the like instead of attaching themetal or the metal compound in advance to the whole surface of thesubstrate. The metal or the metal compound does not necessarily have acompletely continuous film shape, and may be dispersed in a measure.

Then, as shown in FIG. 13B, after forming the antenna 1742, a protectivelayer 1745 is formed over the third interlayer insulating film 1741 soas to cover the antenna 1742. The protective layer 1745 is formed of amaterial which can protect the antenna 1742 when the release layer 1701is etched away later. For example, the protective layer 1745 can beformed by applying a resin of an epoxy type, an acrylate type, a silicontype, or the like which can be dissolved in water or alcohols.

Subsequently, as shown in FIG. 13B, a groove 1746 is formed so as toseparate the respective wireless IC tags. The groove 1746 has such adepth that the release layer 1701 is exposed. The groove 1746 can beformed by dicing, scribing, or the like. If it is not necessary toseparate the wireless IC tags formed over the first substrate 1700, thegroove 1746 is not necessarily formed.

Next, as shown in FIG. 13D, the release layer 1701 is etched away. Here,halogen fluoride is used as the etching gas, and this gas is introducedfrom the groove 1746. For example, ClF₃ (chlorine trifluoride) is used,and the etching is carried out at 350° C. with a flow rate of 300 sccmunder a pressure of 798 Pa for three hours. Nitrogen may be mixed intothe ClF₃ gas. By using halogen fluoride gas such as ClF₃, the releaselayer 1701 can be selectively etched, thereby peeling the firstsubstrate 1700 from the TFTs 1730 to 1732. The halogen fluoride may beeither gas or liquid.

Further, the first substrate 1700 may be peeled by heat treatmentwithout using an etchant. For example, a tungsten (W) film is used asthe release layer 1701 and is subjected to heat treatment, therebyforming a tungsten oxide (WO_(x)) film on the tungsten film.Accordingly, the tungsten oxide film is formed, so that a part betweenthe release layer and the base insulating layer 1702 becomes brittle;thus, the glass substrate can be peeled readily.

Subsequently, as shown in FIG. 14A, the peeled TFTs 1730 to 1732 and theantenna 1742 are attached to a second substrate 1751 using an adhesive1750. The adhesive 1750 is formed with a material which can attach thesecond substrate 1751 with the base insulating film 1702. As theadhesive 1750, various curing adhesives for example, a reactive curingadhesive, a thermosetting adhesive, a photocuring adhesive such as a UVcurable adhesive, an anaerobic adhesive, or the like can be used.

The second substrate 1751 can be formed of a flexible organic materialsuch as paper or plastic.

Next, as shown in FIG. 14B, after removing the protective layer 1745, anadhesive 1752 is applied onto the third interlayer insulating film 1741so as to cover the antenna 1742, and a cover material 1753 is attached.The cover material 1753 can be formed of a flexible organic materialsuch as paper or plastic similarly to the second substrate 1751. Thethickness of the adhesive 1752 may range from, for example, 10 μm to 200μm.

The adhesive 1752 is formed of a material which can attach the covermaterial 1753 with the third interlayer insulating film 1741 and theantenna 1742. As the adhesive 1752, various curing adhesives forexample, a reactive curing adhesive, a thermosetting adhesive, aphotocuring adhesive such as a UV curable adhesive, an anaerobicadhesive, or the like can be used.

Through the above steps, the wireless IC tag is completed. In accordancewith the above manufacturing method, an extremely thin integratedcircuit with a thickness of 0.3 μm to 3 μm, typically about 2 μm, can beformed between the second substrate 1751 and the cover material 1753.

The thickness of the integrated circuit includes not only the thicknessof the semiconductor element itself but also the thicknesses of variousinsulating films and interlayer insulating films formed between theadhesive 1750 and the adhesive 1752. Moreover, the area of theintegrated circuit in the wireless IC tag can be made 5 mm square orless (25 μm² or less), more desirably about 0.3 mm square (0.09 μm²) to4 mm square (16 μm²).

Although this embodiment mode has shown the method for separating thesubstrate and the integrated circuit by etching away the release layerprovided between the integrated circuit and the first substrate 1700having high heat resistance, the method for manufacturing a wireless ICtag of the present invention is not limited to the structure. Forexample, a metal oxide film may be provided between the integratedcircuit and the substrate having high heat resistance and this metaloxide film may be weakened by crystallization, so that the integratedcircuit is peeled. Alternatively, a release layer using an amorphoussemiconductor film containing hydrogen is provided between theintegrated circuit and the substrate having high heat resistance andthis release layer is removed by irradiation with a laser beam, so thatthe substrate and the integrated circuit are separated from each other.Alternatively, the highly heat resistant substrate provided with theintegrated circuit may be eliminated mechanically or etched away withthe use of solution or gas, thereby separating the integrated circuitfrom the substrate.

Although this embodiment mode has shown the example of forming theantenna and the integrated circuit over one substrate, the presentinvention is not limited to this structure. The antenna and theintegrated circuit may be formed over different substrates and may beelectrically connected by being attached to each other later.

Generally, an electric wave frequency of 13.56 MHz, 433 MHz, 860 MHz to960 MHz, or 2.45 GHz is used for RFID (Radio Frequency IDentification).It is very important to form wireless IC tags so as to detect electricwaves with these frequencies for increasing versatility.

The wireless IC tag of this embodiment mode has advantages that anelectric wave is hardly blocked as compared with an RFID tag formed froma semiconductor substrate and attenuation of signals due to the block ofthe electric wave can be prevented. Accordingly, since semiconductorsubstrates are not necessary, the manufacturing cost of the wireless ICtag can be substantially decreased.

Although this embodiment mode has shown the example of peeling theintegrated circuit and attaching the integrated circuit to the flexiblesubstrate, the present invention is not limited to this structure. Forexample, in the case of using a substrate having high temperature whichcan withstand the heat treatment in the manufacturing process of theintegrated circuit, such as a glass substrate, the integrated circuit isnot necessarily be peeled.

Embodiment 8

In this embodiment, a specific example of using a semiconductor devicemanufactured in accordance with the present invention.

FIG. 15A shows a display device including a case 2201, a support 2202, adisplay area 2203, a speaker portion 2204, a video input terminal 2205,and the like. Pixels in the display area 2203 include thin filmtransistors and the EL display device explained in Embodiment 6 may beused for the display area 2203. By manufacturing the thin filmtransistors included in the display area 2203 using the invention,bright display with little defect can be achieved. Further, the displayarea 2203 may have a memory, a driver circuit area, and the like, andthe semiconductor device of the present invention may be applied to thememory, the driver circuit area, and the like. The display area includesvarious display devices in which thin film transistors and variousdisplay media are combined, such as a liquid crystal display deviceusing an electro-optic effect of a liquid crystal, a display deviceusing a luminescent material of electroluminescence or the like, adisplay device using an electron source element, or a display deviceusing a contrast medium whose reflectivity varies in accordance with theapplication of an electric field (also referred to as electronic ink).The display device can be used for all kinds of information displaydevices, such as computers, televisions, information display devicessuch as electronic books, an advertisement display, or a guidancedisplay.

FIG. 15B shows a computer including a case 2211, a display area 2212, akeyboard 2213, an external connection port 2214, a pointing mouse 2215,and the like. Thin film transistors are used in the display area 2212and a CPU, a memory, a driver circuit area, and the like which areprovided on the computer. By applying the semiconductor device inaccordance with the present invention to the display area 2212 and theCPU, the memory, the driver circuit area, and the like which areprovided on the computer, the product performance can be improved andthe drive capacity can be increased.

FIG. 15B shows a mobile phone as a typical example of mobile terminals.This mobile phone includes a case 2221, a display area 2222, anoperation key 2223, and the like. Thin film transistors are used in thedisplay area 2222 and a CPU, a memory, a driver circuit area, and thelike which can be provided on the mobile phone. By applying thesemiconductor device in accordance with the present invention to thedisplay area 2222 and the CPU, the memory, the driver circuit area, andthe like attached to the mobile phone, the product quality can beimproved and the variation in the quality can be decreased. Thesemiconductor devices manufactured in accordance with the presentinvention can be used in electronic devices such as a PDA (PersonalDigital Assistant), a digital camera, and a compact game machine, inaddition to the mobile phone.

FIGS. 15D and 15E show a digital camera. FIG. 15E shows a rear side ofthe digital camera shown in FIG. 15D. This digital camera includes acase 2231, a display area 2232, a lens 2233, operation keys 2234, ashutter 2235, and the like. Thin film transistors are used in thedisplay area 2232, a driver circuit area for controlling the displayarea 2232, and the like. By applying the semiconductor devicemanufactured in accordance with the present invention to the displayarea 2232, the driver circuit area for controlling the display area2232, and other circuits, the product performance can be improved andthe drive capacity can be increased.

FIG. 15F shows a digital video camera including a main body 2241, adisplay area 2242, a case 2243, an external connection port 2244, aremote control receiving portion 2245, an image receiving portion 2246,a battery 2247, an audio input portion 2248, operation keys 2249, aneyepiece portion 2250, and the like. Thin film transistors are used inthe display area 2242 and a driver circuit area for controlling thedisplay area 2242. By applying the thin film transistors manufacturedbin accordance with the present invention to the display area 2242, thedriver circuit area for controlling the display area 2242, and othercircuits, the product performance can be improved and the drive capacitycan be increased.

Moreover, the thin film transistors manufactured in accordance with thepresent invention can be used for a thin film integrated circuit or acontactless thin film integrated circuit device (also referred to as awireless IC tag or an RFID (Radio Frequency IDentification) tag). A thinfilm integrated circuit and a contactless thin film integrated circuitmanufactured by using the manufacturing method shown in anotherembodiment mode can be used for a tag or a memory.

FIG. 16A shows a passport 2301 with a wireless IC tag 2302 attachedthereto. Alternatively, the wireless IC tag 2302 may be embedded in thepassport 2301. Similarly, the wireless IC tag can be attached to orembedded in a driver's license, a credit card, a banknote, a coin, acertificate, a merchandise coupon, a ticket, a traveler's check (T/C), ahealth insurance card, a resident card, a family register, and the like.In this case, only the information showing the authenticity is input inthe wireless IC tag and an access authority is set so that theinformation cannot be read and written improperly. By using the tag inthis way, it is possible to distinguish the forged one and the real one.

Besides, the wireless IC tag can be used as a memory. FIG. 16B shows anexample of embedding a wireless IC tag 2311 in a label to be pasted to apackage of vegetables. Alternatively, the wireless IC tag may beattached directly to the package or embedded in the package. In thewireless IC tag 2311, a production area, a producer, a date ofmanufacture, and a process at a production stage such as a processmethod, a distribution process of the product, a price, quantity, anintended purpose, a shape, weight, an expiration date, each kind ofauthentication information, and the like can be recorded. Informationfrom the wireless IC tag 2311 is received by an antenna portion 2313 ofa wireless reader 2312 and read, and displayed in a display area 2314 ofthe reader 2312. Thus, wholesalers, retailers, and consumers can obtainthe information easily. Moreover, access authority can be set for eachof producers, traders, and consumers. Those who do not have the accessauthority cannot read, write, rewrite, and erase the data in thewireless IC tag.

The wireless IC tag can be used in the following manner. At the payment,information that the payment has been made is written in the wireless ICtag, and the wireless IC tag is checked by a checker provided at an exitwhether or not the information that the payment has been made is writtenin the wireless IC tag. If the IC tag is brought out from the storewithout finishing the payment, an alarm rings. With this method,forgetting of the payment and shoplifting can be prevented.

In consideration of protecting customers' privacy, the following methodis also possible. At the payment at a cash register, any one of thefollowings is conducted; (1) data input in the wireless IC tag arelocked with pin numbers or the like, (2) data themselves input in thewireless IC tag are encrypted, (3) data input in the wireless IC tag areerased, and (4) data input in the wireless IC tag are destroyed. Then, achecker is provided at an exit, and whether any one of (1) to (4) hasbeen conducted or whether the data in the wireless IC tag are notprocessed is checked so that whether the payment has been made or not ischecked. In this way, whether the payment has been made or not can bechecked in the store, and reading out the information in the wireless ICtag against the owner's will outside the store can be prevented.

Several methods for destroying the data input in the wireless IC tag (4)can be given. For example, (a) only data are destroyed by writing one orboth of “0 (off)” and “1 (on)” in at least a part of electronic data inthe wireless IC tag, or (b) current is excessively supplied to thewireless IC tag to physically destroy a part of a wiring in asemiconductor element included in the wireless IC tag.

Since the manufacturing cost of those wireless IC tags mentioned aboveis higher than that of conventionally used barcodes, the cost reductionis necessary. According to the present invention, however, since uniformlaser annealing of a semiconductor film is possible, semiconductordevices with favorable product quality without variation can bemanufactured efficiently, which is effective in the cost reduction.Furthermore, the wireless IC tags can be manufactured so that all of thewireless IC tags have high product quality without variation ofperformance.

As thus described, the semiconductor device manufactured by the presentinvention can be applied in a wide range, and the semiconductor devicemanufactured by the present invention can be applied to electronicdevices in all fields.

1. A method for manufacturing a semiconductor device, comprising thesteps of: splitting a laser beam having a pulse width of 100 fs to 1 nsat a repetition rate of 10 MHz or more into a plurality of split beams;providing an optical path difference having a length corresponding tothe pulse width or more and less than a coherence length between a setof two arbitrary split beams selected from the plurality of split beams;and irradiating a common portion of a semiconductor film with the splitlaser beams in different periods respectively, thereby crystallizing thesemiconductor film.
 2. A method for manufacturing a semiconductordevice, comprising the steps of: splitting a laser beam having a pulsewidth of 100 fs to 1 ns at a repetition rate of 10 MHz or more into aplurality of split beams; providing an optical path difference having alength corresponding to the pulse width or more and less than a lengthcorresponding to a pulse repetition interval between a set of twoarbitrary split beams selected from the plurality of split beams; andirradiating a common portion of a semiconductor film with the set of twoarbitrary split beams in different periods respectively, therebycrystallizing the semiconductor film.
 3. A for manufacturing asemiconductor device, comprising the steps of: splitting a laser beamhaving a pulse width of 100 fs to 1 ns at a repetition rate of 10 MHz ormore into a first laser beam and a second laser beam; delaying thesecond laser beam from the first laser beam; and irradiating a commonportion of a semiconductor film with the first laser beam and the secondlaser beam in different periods respectively, thereby crystallizing thesemiconductor film, wherein the second laser beam is delayed from thefirst laser beam by a range of the pulse width or more and less than apulse repetition interval of the laser beam.
 4. A manufacturing asemiconductor device, comprising the steps of: splitting a laser beamhaving a pulse width of 100 fs to 1 ns at a repetition rate of 10 MHz ormore into a first laser beam and a second laser beam; delaying thesecond laser beam from the first laser beam; and irradiating a commonportion of a semiconductor film with the first laser beam and the secondlaser beam in different periods respectively, thereby crystallizing thesemiconductor film, wherein the semiconductor film is irradiated withthe second laser beam while the semiconductor film is melted byirradiation with the first laser beam.
 5. A laser irradiation apparatuscomprising: means for splitting a laser beam emitted from an oscillationsource, which has a pulse width of 100 fs to 1 ns at a repetition rateof 10 MHz or more into a plurality of split laser beams; means forproviding an optical path difference between a set of two arbitrarysplit laser beams selected from the plurality of split laser beams; andmeans for irradiating an object to be irradiated with the set of twoarbitrary split laser beams in different periods respectively, whereinthe means for providing an optical path difference provides an opticalpath difference having a length corresponding to the pulse width or morebetween the plurality of split laser beams.
 6. A laser irradiationapparatus comprising: means for splitting a laser beam emitted from anoscillation source, which has a pulse width of 100 fs to 1 ns at arepetition rate of 10 MHz or more into a plurality of split beams; amedium, which does not absorb the laser beam, for providing an opticalpath difference between a set of two arbitrary split laser beamsselected from the plurality of split laser beams; means for irradiatingan object to be irradiated with the set of two arbitrary split laserbeams in different periods respectively, wherein the medium provides anoptical path difference having a length corresponding to the pulse widthor more between the plurality of split laser beams.
 7. A laserirradiation apparatus according to claim 6, wherein the medium is acrystal of quartz, glass, water, or fluoride.